Field of the Invention
The disclosure relates to an image pickup apparatus, a camera system, and an image processing apparatus.
Description of the Related Art
Various correction processing methods, which correct a captured image by treating an image as a signal value, have been proposed with digitalization of information. When an object photographed by a digital camera is imaged, an acquired image degrades not a little especially by aberrations of an image pickup optical system.
Image blurring components include spherical aberration, coma aberration, curvature of field, and astigmatism of an optical system. The presence of the image blurring components caused by these types of aberrations means that when a light beam emitted from one point does not suffer from any aberration or diffraction, the light beam fails to converge to another point, being minutely spread around the another point. Such a minutely-spread distribution is represented by point spread function (PSF). Image blurring components is not out of focus, but deteriorations by influence of aberrations of the above optical system when being in focus. In addition, color bleeding in a color image, which is caused by axial chromatic aberration, a spherical aberration in color, and coma aberration in a color of the optical system, is generated due to a blur in each wavelength of light. Moreover, a color shift in a lateral direction caused by transverse chromatic aberration of the optical system is a position blur or a phase blur due to an image magnification in each wavelength of light.
An optical transfer function (OTF) obtained by the Fourier transform of the PSF is frequency component information on an aberration and represented by a complex number. The absolute value of the OTF, i.e., an amplitude component, is referred to as a “modulation transfer function” (MTF), and a phase component is referred to as a “phase transfer function” (PTF), respectively. The MTF and the PTF are frequency characteristics of the amplitude component and the phase component of image deterioration caused by an aberration, respectively. These are represented by the following expression with the phase component being defined as a phase angle. Symbols Re (OTF) and Im (OTF) represent a real part and a imaginary part of the OTF, respectively.PTF=tan−1(Im(OTF)/Re(OTF))
Because the optical transfer function OTF of the image pickup optical system deteriorates the amplitude component and the phase component of an image, each point of an object asymmetrically blurs as coma aberration in the deteriorated image.
Transverse chromatic aberration occurs in a situation in which an imaging position is shifted due to a difference in the formation ratio depending on each light wavelength and the formed image is obtained as an RGB color component, for example, according to spectral characteristics of the image pickup apparatus. This means that imaging positions of R, G, and B components are shifted to each other, leading to an imaging position shift in each color component depending on a wavelength, that is, a spread of an image due to a phase shift. Thus, transverse chromatic aberration is not just the color shift being a parallel shift exactly, but the color shift is described with the same mean as transverse chromatic aberration unless otherwise noted.
A method of correcting a deterioration of the MTF and the PTF utilizing information of the OTF of the image pickup optical system is known. This method is generally called image restoration or image reconstruction, and the method of convoluting an image restoration filter with reverse characteristics of an optical transfer function OTF with respect to an input image is known as one example of the image restoration methods. Japanese Patent Laid-open No. 2011-205231 discloses an invention to accelerate an image restoration process. Because the image restoration process performs a convolution process relative to an image restoration filter, which is a two-dimensional filter, a size of the image restoration filter influences processing speed. Accordingly, when off-axis aberrations, such as curvature of field, are large, generating the image restoration filter of a state where a center of the image is in-focus expands the image restoration filter at the circumferential part of the image, and slows the process. Thus, Japanese Patent Laid-open No. 2011-205231 moves a focusing surface to minify the image restoration filter over the entire image height, and thus decreases the image restoration filter over the entire image to accelerate the image restoration process.
Conventional image restoration filters for the image restoration process, which are generated on the assumption that an image pickup environment is fixed, fails to correspond to a change of the image pickup environment, such as atmospheric temperature and humidity. Thus, in an optical system including an imaging lens, such as a projection lens, arranged near a high heat source, such as a lamp, or a plastic lens whose optical characteristics enormously change by atmospheric temperature and humidity, optical characteristics enormously change by the image pickup environment, and thus the image restoration process cannot be performed appropriately. Japanese Patent Laid-open No. 2011-205231 fails to disclose a method regarding reduction of image deteriorations caused by the image pickup environment.